The invention relates to a control device for influencing a flow of energy in a load circuit between an electrical voltage source and an electrical load, having a semiconductor switch comprising a conductive section which is formed between an input connection and an output connection, which can be looped into the load circuit, and has an electrical resistance adjustable by means of an electrical potential which can be applied to a control connection associated with the semiconductor switch, and having a control circuit which is coupled to the control connection and comprises a freewheeling means, in particular a freewheeling diode, connected in parallel to the load. The invention also relates to a method for actuating a semiconductor switch.
In a switchable electrical load circuit there is usually a direct or magnetically coupled series connection of a voltage source, a semiconductor switch and a load. In order to avoid overloading of the semiconductor switch, a freewheeling diode is connected in parallel to the inductance-prone load, the inductance of which may be determined wholly or partly by the electrical wiring of the load circuit.
A semiconductor switch, for example in the form of a MOSFET or IGBT, is controlled with the aid of a gate-source voltage, ensuing from charging and discharge currents applied to the gate and generating a control charge as a result of the linked internal capacitances of the semiconductor switch. In the case of low-impedance semiconductor switches, the supply potential is substantially applied at the connection point between the semiconductor switch and the directly or magnetically coupled load with the freewheeling diode. If the gate-source voltage is now reduced, in order to bring the semiconductor switch into a blocking state, the internal resistance of the transistor rises and the power converted into heat inside it becomes greater. This is associated with the fact that the current reduces only very slightly due to the effect of the inductance in the load circuit. It is therefore necessary that the freewheeling diode can take over the current flow as soon as possible, before the amount of heat developed in the transistor results in the destruction of the latter.
From EP 0 756 782 B1 it is known, depending on the voltage at the freewheeling semiconductor element, that controllable breaking current sources for continuous reduction of the level of the breaking current may be provided up to a minimum value. Here the minimum value is reached substantially with a voltage value of 0 volts of the reducing voltage at the freewheeling semiconductor element.
DE 40 13 997 discloses, depending on a threshold value of the voltage at the freewheeling semiconductor element, a controllable switch-on current source which is provided for immediate changeover of the amount of switch-on current to a high value, when the threshold value indicates that the freewheeling semiconductor element has switched-off.
The problem of the present invention therefore lies in the further development of the control device and the method for actuating a semiconductor switch so as to ensure, when current is switched on or off, that there is reduced power loss combined with simultaneous more rapid switching of the semiconductor switch.